The tobacco hornworm, Manduca sexta, and the fruit fly, Drosophila, are favorable experimental material in which to examine the developmental effects of hormones on the CNS. During larval life, both species show extensive neurogenesis which can be quantified by incorporation of bromodeoxyuridine into the DNA of dividing cells followed by immunocytochemistry. With this tool, the extrinsic factors regulating neurogenesis in normal larvae can be determined. Cell proliferation mutants in Drosophila will then be screened for those which interfere with various aspects of neurogenesis. Culturing of neuroblasts from mutant and wild-type animals will further clarify the role of intrinsic and extrinsic factors in regulating neurogenesis. The mechanisms by which hormones bring about the programmed death of mature neurons will be examined. Drosophila will be screened for mutants that interfere with this programmed death. Also, labeled steroids and autoradiography will be used in the moth to further define the relationship of hormone target cells to cells that die. During larval life, the insect CNS produces discrete nests of cells, each arising from a single, identifiable neuroblast. After their birth the cells arrest at a point before extensive process outgrowth and transmitter expression. Later, these cells then mature into functional neurons during metamorphosis in response to circulating ecdysteroids. A series of in vivo and in vitro experiments will examine the regulation of differentiation of these cells in response to the steroid signal. Neurons produced by specific lineages will be defined by antibodies against various transmitters. Using chemical ablation techniques, neuroblasts will be killed at various times to allow examination of the relationship of lineage size to the subsequent differentiation of its members. Specific lineage groups will be cultured and their differentiation in vitro in response to steroids will be compared to their normal fate in vivo.